1. Field
The present disclosure generally relates to vacuum forming and, in particular, controlling the deformation of a part during the vacuum forming process.
2. Description of the Related Art
Components made from fiber-reinforced composite material often use a lightweight core material covered with surface layers of the composite material. This construction can provide a high strength-to-weight ratio which is particularly advantageous in aerospace applications. Foam materials such as polystyrene, polyurethane, and polymethacrylimide are commonly used as cores, depending on the resins and operational requirements. Foams are available in a variety of densities, which vary with the compressive strength of the foam, and may be open-cell or closed-cell, depending on the material.
Some manufacturing techniques shape the foam core and then apply the composite materials to the core. Foam blanks are available in a variety of shapes and sizes, including sheets and blocks in thicknesses of a few millimeters up to 10 centimeters or more. A typical aerospace-grade foam is Rohacell® A (available from Evonik Röhm GmbH, Performance Polymers, 64293 Darmstadt, Germany) that can be formed after being heated to approximately 350 degrees Fahrenheit (F).
One drawback to thermoforming foam is that it is impractical to manually handle the material while the foam is at the working temperature. One existing method of thermoforming a foam blank around a tool is to place the blank over the tool and enclose the blank and the tool in a sealed bag, heat the tool and foam blank to the working temperature, and then create a vacuum within the bag. The external air pressure applies a uniformly distributed force that forms the foam around the tool. This is a delicate process, however, that is subject to a high degree of variability, and the rate of forming is sensitive to the vacuum level. It is not uncommon for the foam blank to crack during the forming process when the foam is formed too quickly.